Thin film piezoelectric resonator and method of manufacturing the same

ABSTRACT

A thin film piezoelectric resonator includes a substrate having a cavity; a first electrode extending over the cavity; a piezoelectric film placed on the first electrode; and a second electrode placed on the piezoelectric film, the second electrode having a periphery partially overlapping on the cavity and tapered to have an inner angle of 30 degrees or smaller defined by a part of the periphery thereof and a bottom thereof.

TECHNICAL FILED

This invention relates to a thin film piezoelectric resonator and amethod of manufacturing the same, and more particularly relates to athin film piezoelectric resonator which uses axial vibrations in adirection of the thickness of a thin piezoelectric film, and the methodof manufacturing such a thin film piezoelectric resonator.

BACKGROUND ART

Along with a remarkable breakthrough in the field of wirelesscommunications, a variety of developments have been in progress in orderto accelerate information transmission. With the wirelesscommunications, frequency bands of approximately 2 GHz are in wide usein order to cope with the introduction of PHS systems, third generationcellular phones, wireless LAN, and so on. Further, the number of usersand wireless terminals are extensively increased. The higher theinformation transmission speeds, the higher the carrier frequencies.Wireless LAN systems operating on a 5 GHz frequency band are now inbusiness use.

There are strong demands on miniaturization of communication devicesoperating on high frequency bands. Especially, with a personal computer(PC), a communication device is realized by a PC card which should be asthin as approximately several millimeters.

Generally, a wireless communication device in the shape of the PC cardincludes an RF front end which processes radio frequencies, and a baseband (BB) unit which processes digital signals. The base band unit ispreferably an LSI chip made of a silicon (Si) substrate, and can bethinned to 1 mm or less.

The RF front end amplifies and coverts high frequencies as analogsignals, and includes a number of passive components such as oscillatorsand filters. It is technically difficult to constitute the RF front endonly by an LSI chip because the RF front end has a very complicatedstructure. The filters are either dielectrics or LC filters. The filterscan filter high frequency signals using passband characteristics of acavity resonator or an LC circuit, and is essentially difficult to bedownsized and to be thinned to several millimeters or less. In otherwords, communication devices operating on high frequency bands havelimitations on their miniaturization.

In order to overcome the foregoing problem, Japanese Patent Laid-OpenPublication No. 2000-069,594 proposes a film bulk acoustic waveresonator (FBAR) which has attracted attentions, for example. In theFBAR, a thin piezoelectric film made of aluminum nitride (AlN) or zincoxide is sandwiched between lower and upper electrodes. The thinpiezoelectric film is placed over a cavity in a substrate. The resonatorlets frequencies resonate along the thickness of not only the lower andupper electrodes which are in contact with an air layer but also thepiezoelectric film. The foregoing thickness of 0.5 μm to several μmwhich can be accomplished by a film making process is suitable tofrequencies of several GHz. Therefore, resonators compatible with highfrequencies in GHz bands can be easily manufactured.

For instance, two thin film piezoelectric resonators are connected inseries, and one thin film piezoelectric resonator is connected inparallel with the two thin film piezoelectric resonators, therebyobtaining a ladder-shaped filter. With a passband filter, the centralfrequency of the series-connected this film piezoelectric resonators andthat of the parallel-connected thin film piezoelectric resonator areslightly different. Therefore, the resonance frequency of theparallel-connected thin film piezoelectric resonator is adjusted to beequal to that of the series-connected thin film piezoelectricresonators, for instance.

The foregoing thin film piezoelectric resonator can be produced usingthe film making process which is used to form a thin film on asubstrate, and can be miniaturized. Especially, a general purpose filtercan be easily made as thin as 1 mm or less, which is usually verydifficult. Further, the substrate may be made of Si, which enables thethin film piezoelectric resonator to be produced by a semiconductormanufacturing process. Still further, the thin film piezoelectricresonator is reliably compatible with a transistor, IC, LSI and so on,and can have them mounted thereon.

However, there are the following new problems when a high frequencymodule is made using the thin film piezoelectric resonator on which atransistor, IC, LSI and so on are mounted.

The thin film piezoelectric resonator operates on bulk standing waveswhich are generated in a direction extending along the thickness of thepiezoelectric film and produces resonance. However, lateral modestanding waves are generated at an edge of an electrode and an edge of apiezoelectric film. Such lateral mode standing waves have specificvalues. As a result, a lateral mode standing wave is generated. Awavelength of the lateral mode standing wave differs from that of thebulk wave. When combined with the bulk wave, the lateral mode standingwave causes a variety of parasitic vibration modes (spuriousvibrations). If spurious vibrations are caused, ripples are generated,which fluctuate high frequency signal characteristics (Smith chart).This phenomenon extensively deteriorates resonance performance of thethin film piezoelectric resonator, or makes the resonance performancevariable.

In order to overcome the foregoing problem, it has been proposed tosuppress the lateral mode standing wave by making a contour of an upperelectrode of a thin film piezoelectric resonator in the shape of anirregular polygon, as shown in FIG. 14 of the accompanying drawings.However, since such an upper electrode 104 becomes large, it isimpossible to miniaturize a filter including thin film piezoelectricresonators.

Referring to FIG. 15 and FIG. 16, a thin film piezoelectric resonator100 includes an upper electrode 104, a substrate 101 having a cavity101H, a lower electrode 102 extending over the cavity 101H, and a thinpiezoelectric film 103 on the lower electrode 102. The upper electrode104 is present over the piezoelectric film 103, and has a damping layer105 at its one end. The damping layer 105 damps the lateral modestanding wave. However, a new process for making the damping layer 105should be added to a process for making the thin film piezoelectricresonator 100. This not only increases the number of manufacturingprocesses but also reduces an yield of the thin film piezoelectricresonator 100. Further, the damping layer 105 should be aligned with theedge of the upper electrode 104. However, a sufficient processing margincannot be secured.

DISCLOSURE OF INVENTION

The invention is aimed at providing a thin film piezoelectric resonatorwhich can overcome the foregoing problems of the related art, improveresonating performance thereof, and be miniaturized.

Further, the invention is aimed at providing a method of manufacturingthe thin film piezoelectric resonator which can reduce the number ofmanufacturing steps, improve the yield, and secure a sufficientprocessing margin.

A first aspect of the embodiment of the invention relates to a thin filmpiezoelectric resonator which includes a substrate having a cavity; afirst electrode extending over the cavity; a piezoelectric film placedon the first electrode; and a second electrode placed on thepiezoelectric film. The second electrode has a part of a periphery whichoverlaps on the cavity, is tapered, and has an inner angle of 30 degreesor smaller defined by a part of the periphery thereof and a bottomthereof.

A second aspect of the embodiment of the invention relates to a thinfilm piezoelectric resonator includes a substrate having a cavity; afirst electrode extending over the cavity; a piezoelectric film placedon the first electrode; a second electrode placed on the piezoelectricfilm and having a part of a periphery thereof which overlaps on thecavity; and an insulator placed on the second electrode and thepiezoelectric film where the second electrode is absent, and being thinon the center and thick on the periphery of the second electrode.

A third aspect of the embodiment of the invention relates to a thin filmpiezoelectric resonator includes a substrate having a cavity; a firstelectrode extending over the cavity; a piezoelectric film placed on thefirst electrode; a second electrode placed on the piezoelectric film andhaving a periphery which overlaps on the cavity, is tapered, and has aninner angle of 30 degrees or smaller defined by a part of the peripherythereof and a bottom thereof; and an insulator placed on the secondelectrode and the piezoelectric film where the second electrode isabsent, and being thin on the center of the second electrode and thickon the periphery of the second electrode.

A fourth aspect of the embodiment of the invention relates to a thinfilm piezoelectric resonator includes a substrate having a cavity; afirst electrode extending over the cavity; a piezoelectric film placedon the first electrode; a second electrode placed on the piezoelectricfilm and having a periphery which overlaps on the cavity, and aninsulator placed on the second electrode and the piezoelectric filmwhere the second electrode is absent, and having the thickness varyingon the piezoelectric film and on the periphery of the second electrode.

A fifth aspect of the embodiment of the invention relates to a thin filmpiezoelectric resonator includes a substrate having a cavity; a firstelectrode extending over the cavity; a piezoelectric film placed on thefirst electrode; a second electrode placed on the piezoelectric film andhaving a periphery which overlaps on the cavity; and an insulator placedon the second electrode and the piezoelectric film where the secondelectrode is absent and having the thickness gradually varying on thepiezoelectric film and on the periphery of the second electrode.

A final aspect of the embodiment of the invention relates to a method ofmanufacturing a thin film piezoelectric resonator. The method includesmaking a cavity in a substrate; making a first electrode over thecavity; making a piezoelectric film on the first electrode; making anelectrode forming layer on the piezoelectric film; making a photo-resistlayer on the electrode forming layer, the photo-resist layer overlappingon the cavity; tapering an edge of the photo-resist layer, the taperedphoto-resist layer having an acute angle and serving as a mask; andpatterning the electrode layer using the mask in order to make a secondelectrode, and transferring a shape of the tapered edge of the mask ontoan end of the second electrode, the tapered edge of the second electrodehaving an acute inner angle.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a cross section of a thin film piezoelectric resonatoraccording to a first embodiment of the invention, taken along line F1-F1shown in FIG. 2;

FIG. 2 is a top plan view of the thin film piezoelectric resonator ofFIG. 1;

FIG. 3 is an enlarged cross section of an essential part of the thinfilm piezoelectric resonator of FIG. 1;

FIG. 4 shows the relationship between an angle of an edge of a secondelectrode in the thin film piezoelectric resonator of FIG. 3 and ananti-resonance point;

FIG. 5 shows high frequency characteristics of the thin filmpiezoelectric resonator of FIG. 3;

FIG. 6 is a cross section showing how the thin film piezoelectricresonator of FIG. 1 and FIG. 2 is made in a first manufacturing process;

FIG. 7 is a cross section showing how the thin film piezoelectricresonator is made in a second manufacturing process;

FIG. 8 is a cross section showing how the thin film piezoelectricresonator is made in a third manufacturing process;

FIG. 9 is a cross section showing how the thin film piezoelectricresonator is made in a fourth manufacturing process;

FIG. 10 is an enlarged cross section of an essential part of a thin filmpiezoelectric resonator according to a second embodiment;

FIG. 11 is an enlarged cross section of an essential part of a thin filmpiezoelectric resonator according to a third embodiment;

FIG. 12 shows the relationship between a distance between an edge of asecond electrode and a stepped portion of an insulator and ananti-resonance point of the thin film piezoelectric resonator of FIG.13;

FIG. 13 is an enlarged cross section of an essential part of a thin filmpiezoelectric resonator according to a fourth embodiment;

FIG. 14 shows a contour of an upper electrode in a thin filmpiezoelectric resonator in the related art;

FIG. 15 is a cross section of the thin film piezoelectric resonator ofFIG. 14, taken along line F14-F14 in FIG. 16; and

FIG. 16 is a top plan view of the thin film piezoelectric resonator ofFIG. 15.

BEST MODE FOR CARRYING OUT THE INVENTION First Embodiment [Configurationof Thin Film Piezoelectric Resonator]

Referring to FIG. 1 to FIG. 3, a thin film piezoelectric resonator 1(called the “piezoelectric resonator 1”) in this embodiment includes asubstrate 2 having a cavity 2H; a first electrode (lower electrode) 5extending over the cavity 2H; a piezoelectric film 6 extending on firstelectrode 5; and a second electrode 7 extending partly on thepiezoelectric film 6. A periphery 71 of the second electrode 7 overlapson the cavity 2H, and is tapered to have an inner angle θ of 30 degreesor smaller defined by a part of the periphery thereof and a bottomthereof, e.g., the inner angle θ is set between 15 degrees and 30degrees in the first embodiment.

The substrate 2 is made of silicon in this embodiment. The cavity 2H isin the shape of a rectangle, for example. When an acoustic reflector isused, it is placed in the cavity 2H, which is made by digging a part ofthe substrate 2.

The first electrode 5 extends over the cavity 2H and on the substrate 2.A part of the first electrode 5 which does not extends over the cavity2H and is present on the substrate 2 functions as an outgoing line. Thefirst electrode 5 is mainly made of an aluminum (Al) film or an aluminumalloy film, for example, and has a dual structure, i.e., a lower part ofthe first electrode 5 has the amorphous structure in order to improvethe orientation of the piezoelectric film 6.

The piezoelectric film 6 is placed on the first electrode 5, and extendsabove the cavity 2H. The piezoelectric film 6 is preferably made ofaluminum nitride (AlN), for example.

The second electrode 7 extends on the piezoelectric film 6 and above thecavity 2H. A part of the second electrode 7 which does not extend abovethe cavity 2H functions as an outgoing line on the piezoelectric film 6.The second electrode 7 is preferably made of molybdenum (Mo).

In the piezoelectric resonator 1, the first electrode 5, piezoelectricfilm 6 and second electrode 7 which bridges over the cavity 2H serve asan exciter. The exciter can be vibrated by applying a voltage across thefirst and second electrodes 5 and 7, so that the piezoelectric resonator1 can have resonating characteristics.

A first wiring 8A extends over the substrate 2 but not over the cavity2H, is electrically connected to the first electrode 5 via a dummy pad3, and is preferably made of Mo, for example. A second wiring 8B extendsover the substrate 2 but not over the cavity 2H, is electricallyconnected to the second electrode 7.

The dummy pad 3 is conductive, has etching selectivity to the first andsecond electrodes 5 and 6, and is placed where a lead wiring of thefirst electrode 5 (the lead wiring being flush with the first electrode5) and the periphery of the piezoelectric film 6 are present. The dummypad 3 is structured as a link-up wiring, enables the patterning of thepiezoelectric film 6, prevents the disconnection of the lead wiring(first electrode 5) and the first wiring 8A, and assures the electricconnection between the lead wiring and the first wiring 8A. The dummypad 3 is preferably made of Mo, for example. The dummy pad 3 preferablyhas an inner angle between 30 degrees and 60 degrees in order to improvestep coverage of the first wiring 8A on the dummy pad 3.

[Characteristics of Piezoelectric Resonator]

FIG. 3 is an enlarged cross section of an essential part of the thinfilm piezoelectric resonator of FIG. 1, and FIG. 4 shows therelationship between an angle of an edge of a second electrode in thethin film piezoelectric resonator of FIG. 3 and an anti-resonance point.

In the first embodiment, the piezoelectric resonator 1 alleviatesenvironmental conditions by controlling the edge of the second electrode7 as a stationary end, and efficiently dispersing frequencies ofstanding waves, which suppresses spurious vibrations. Although it isimpossible to completely remove standing waves, it is possible toextensively reduce the spurious vibrations in the Smith chart.

Referring to FIG. 3, the length of the mildly tapered edge 71 of thesecond electrode 7 is very important, and should be as long as bulkwaves of the piezoelectric film 6. For this purpose, the inner angle θof the edge of the second electrode 7 should be 30 degrees or smallertaking a processing margin into consideration.

Further, when the inner angle θ is 30 degrees or less and is beinggradually reduced below 30 degrees, the anti-resonance point is abruptlyraised. Refer to FIG. 4. The higher the anti-resonance point, the lessthe spurious vibrations. On the other hand, since errors of processingprecision of the edge 71 of the second electrode 7 are increased duringfabricating, it not preferable that the inner angle θ is 5 degrees orsmaller. Therefore, the inner angle θ is 15 degrees or larger which isnot practically problematic in view of the processing precision.

The piezoelectric resonator 1 has frequency characteristics as shown inFIG. 5. In FIG. 5, the abscissa represents frequencies while theordinate represents the impedance. The letter “B” denotes frequencycharacteristics when the edge of the second electrode 7 is worked to bevertical (θ=90 degrees). In this case, the high frequencycharacteristics include remarkable spurious vibrations. On the otherhand, the letter “A” denotes frequency characteristics when the edge ofthe second electrode 7 is worked to have the inner angle θ of 30 degreesor smaller. The frequency characteristics do not include the spuriousvibrations.

[Method of Manufacturing Piezoelectric Resonator]

The piezoelectric resonator 1 is manufactured as shown in FIG. 6 to FIG.9. First of all, the substrate is prepared. The Si subject is preferablyused.

Referring to FIG. 6, the dummy pad 3 is formed on the substrate 2 (theupper surface in FIG. 6), at a position where the lead wiring of thefirst electrode 5 and the periphery of the piezoelectric film 6 overlap.The dummy pad 3 is made of an Mo film prepared by the sputteringprocess. The Mo film is patterned using a mask prepared by thephotolithography process.

Thereafter, a first electrode forming layer and a piezoelectric filmforming layer are made in succession on the substrate 2 including thedummy pad 3 in a high vacuum system (refer to FIG. 7). In accordancewith the frequency of the W-DMA specification, a 250 nm—thick Al alloyfilm is used for the first electrode forming layer. A 1700 nm—thick AlNfilm is used for the piezoelectric film forming layer.

Referring to FIG. 7, the piezoelectric film forming layer is patterned,thereby completing the piezoelectric film 6. Then, the first electrodeforming layer is patterned, completing the first electrode 5. Thepatterning is conducted by the reactive dry etching process in whichchlorine (Cl) is used, and by using masks prepared by thephotolithography process.

Next, a second electrode forming layer is formed all over the substrate2 including the piezoelectric film 6 (refer to FIG. 8). A sputtered Mofilm having the thickness of 250 nm is used as the second electrodeforming layer. Thereafter, a photo-resist film is formed on the secondelectrode forming layer at a proposed position of the second electrode 7and the lead wiring. The photolithography process is used in this case.The photo-resist film is baked for approximately 10 minutes to 20minutes at a temperature of 150° C. A mask (etching mask) 75 shown by abroken line in FIG. 8 is made using the photo-resist film. An edge ofthe mask 75 is tapered similarly to the edge 71 of the second electrode7 since the mask 75 is baked. In short, the inner angle θ is preferablybetween 15 degrees and 30 degrees.

The second electrode forming layer is patterned using the mask 75,thereby completing the second electrode 7. The tapered edge of the mask75 is copied onto the edge 71 of the second electrode 7, so that theinner angle θ of the edge 71 is made within the foregoing range. Themask 75 is then removed.

The first wiring 8A (which is electrically connected to the firstelectrode 5 via the dummy pad 3) and the second wiring 8B (which isconnected to the second electrode 7) are made in succession in the samemanufacturing step (refer to FIG. 9).

Thereafter, the substrate 2 has its rear surface etched toward its frontsurface, thereby making the cavity 2H. In this state, the piezoelectricresonator 1 of the first embodiment is completed.

EXAMPLE

A specific example of the piezoelectric resonator 1 of the firstembodiment will be described with reference to FIG. 1 to FIG. 3.

In the piezoelectric resonator 1, the first electrode 5 has a dualstructure in which the lower part thereof is amorphous in order toimprove the orientation of the piezoelectric film 6, i.e., AlN.Resonance characteristics can be improved by controlling the orientationof AlN. The orientation of the sputtered AlN film is controlled to be1.5 degrees or less with respect to the rocking curve of X rays.Further, the stress of AlN is also controlled in order to stabilize thecomponents bridging the cavity 2H. Specifically, the substrate 2 issubject to the high speed RIE from its rear surface in order to make thecavity 2H. The first and second wirings 8A and 8B are made of gold (Au)or Al.

The piezoelectric resonator 1 operates on a 2 GHz band, has anelectrical-mechanical coupling constant of 6.7%, and a resonance Q valueof 800. During the manufacturing process, an in-plane distribution of Siwafer is excellent, and the foregoing characteristics can be reliablyreproduced on a 6-inch wafer.

As described above, in the piezoelectric resonator 1, the edge 71 of thesecond electrode 7 is tapered to have the inner angle θ of between 15degrees and 30 degrees, which prevents the second electrode 7 fromhaving the irregular polygonal shape, improves the resonancecharacteristics, and accomplish the downsizing.

Further, with the manufacturing method of the piezoelectric resonator 1,the photo-resist film which is on the second electrode 7 and overlaps onthe cavity 2H is made, so that the mask 75 is formed by tapering theedge of the photo-resist film in order to make the inner angle acute.The second electrode forming layer is patterned using the mask 75,thereby making the second electrode 7. The tapered edge of the mask 75is copied onto the edge of the second electrode 7. The inner angle θ ofthe edge of the second electrode 7 is between 15 degrees and 30 degrees.The edge of the mask 75 is only baked after the photo-resist film ispatterned. Therefore, it is not necessary to make a new damper layer,which is effective in reducing the number of manufacturing steps andimproving the manufacturing yield.

Second Embodiment

A second embodiment is intended to promote the suppression of thespurious vibrations in the piezoelectric resonator of the firstembodiment, and to provide a piezoelectric resonator which is reliablefor a long period of time.

Referring to FIG. 10, the piezoelectric resonator 1 of this embodimentincludes an insulator (passivation film) 9 which has a dielectricconstant different from that of the second electrode 7, and uniformthickness. The insulator 9 extends over the second electrode 7 includingthe edge 71, and the piezoelectric film 6. In short, the insulator 9 isin direct contact with the second electrode 7. The insulator 9 ispreferably made of a silicon-nitride film (Si₃N₄) which is prepared bythe CVD process and is approximately 2 nm to 50 nm thick. Alternatively,the insulator 9 may be made of a silicon-oxide film (SiO₂), an AlN filmand so on which have dielectric constants different from that of thesecond electrode 7. Further, the insulator 9 may be prepared by thesputtering process or by using an electron gun (E gun) so long as thestepped part is sufficiently covered and the film stress is allowable.

The insulator 9 protects the second electrode 7, piezoelectric film 6and so on of the piezoelectric resonator 1 against the aging caused bythe oxidization. Specifically, the insulator 9 extends over the secondelectrode 7 and the piezoelectric film 6, and prevents them from beingexposed to the air. Therefore, frequencies of standing waves can beeffectively dispersed, which more extensively suppresses spuriousvibrations. The insulator 9 is formed on the second electrode 7 and thepiezoelectric film 6, and does not need any process such as thepatterning, which improves the processing margin in the manufacturingprocess.

Third Embodiment

In a third embodiment, the cross sectional shape of the edge 71 of thesecond electrode 7 and the shape of the insulator 9 of the secondembodiment are modified. The insulator 9 extends over the secondelectrode 7.

Referring to FIG. 11, a piezoelectric resonator 1 includes a substrate 2having a cavity 2H; a first electrode 5 extending over the cavity 2H; asecond electrode 7 whose edge 71 overlaps on the cavity 2H; apiezoelectric film 6; and an insulator 9. The insulator 9 extends overthe second electrode 7 and a part of the piezoelectric film 6. Theinsulator 9 is thin on the center of the second electrode 7, and isthick on the periphery of the second electrode 7.

The cross sectional shape of the edge 71 of the second electrode 7 ofthis embodiment differs from the cross sectional shapes of the edges 71of the second electrodes 7 in the first and second embodiments. In thisembodiment, the edge 71 has the inner angle θ is approximately 90degrees.

The insulator 9 includes a first insulating element 9A and a secondinsulating element 9B. The first insulating element 9A extends over thepiezoelectric film 6 and the periphery of the second electrode 7, and isthick on the edge of the second electrode 7. The second insulatingelement 9B extends over the center of the second electrode 7 and is thinthereon. The first and second insulating elements 9A and 9B are made inthe same manufacturing step by partially etching an insulating film(i.e., on the center of the second electrode 7).

The thickness of the first insulating element 9A varies on thepiezoelectric film 6 (a point 9 a) [extending over the cavity 2H], at aborder (a point 9 b) between the piezoelectric film 6 and the secondelectrode 7, and at the periphery (a point 9 c) of the second electrode7. In other words, effective thicknesses ta, tb and tc of the firstinsulating element 9A are increased at the points 9 a, 9 b and 9 c. Theinsulator 9 seems to be thickened because the thickness of the secondelectrode 7 is added at the points 9 b and 9 c. By accuratelypositioning the edge 71 of the second electrode 7 and determining thepositions where the second insulating element 9A changes its thickness,it is possible to effectively disperse the frequencies of standing wavesand suppress spurious vibrations. The suppression of the spuriousvibrations extensively depends upon the border between the first andsecond insulating elements 9A and 9B.

FIG. 12 shows the relationship between an anti-resonance point and adistance L from the 1 edge 71 of the second electrode 7 to the border ofthe first and second insulating elements 9A and 9B. In FIG. 12, theabscissa denotes the distance L while the ordinate denotes theanti-resonance point. When the second insulating element 9A ispositioned in a range (within the distance L) of 1 μm to 10 μm, theanti-resonance point is increased while spurious vibrations aresuppressed. Especially, when the distance L is 2 μm to 5 μm, spuriousvibrations are most suppressed. The inventors have conductedexperiments, and have noted that spurious vibrations are mostextensively suppressed when the distance L is 3 μm.

Fourth Embodiment

In this embodiment, a thin film piezoelectric resonator 1 is acombination of the thin film piezoelectric resonators 1 of the secondand third embodiments.

Referring to FIG. 13, the piezoelectric resonator 1 includes a substrate2 having a cavity 2H, a first electrode 5 spanning over the cavity 2H, apiezoelectric film 6 on the first electrode 5, a second electrode 7, andan insulator 9. The second electrode 7 has a periphery 71 which overlapsover the cavity 2H, and is tapered by the angle θ of 30 degrees orsmaller. The insulator 9 extends over the second electrode 7 and thepiezoelectric film 6, and is thicker on the piezoelectric film 6 and theperiphery of 71 of the second electrode than on the second electrode 7.Specifically, the insulator 9 has the thickness gradually varying on thepiezoelectric film 6 and the second electrode 7.

The piezoelectric resonator 1 of this embodiment is as advantageous andeffective as those of the second and third embodiments when they arecombined.

Other Embodiments

Although the invention has been described with respect to someembodiments thereof, it will not be understood by those skilled in theart that various other modifications are possible. In the foregoingembodiments, the thin film piezoelectric resonators are described to beapplied to the frequency band of 2 GHz. Alternatively, the invention isapplicable to piezoelectric resonators operating on frequency bands of800 MHz to 5 GHz.

The foregoing descriptions relate to the thin film piezoelectricresonators which are used to constitute filters. Alternatively, theinvention is applicable to constituting voltage-controlled oscillators.

Still further, the piezoelectric resonator is assumed to have the FBARstructure. Alternatively, the piezoelectric resonator may have the SMR(Surface-mounted Resonator) structure having a sound reflecting layer.

As described so far, the present invention provides the thin filmpiezoelectric resonator which can not only improve resonatingcharacteristics but also can be downsized.

Further, the present invention provides the method of manufacturing thethin film piezoelectric resonator. The method improves the yield andassures sufficient processing margins while reducing the number ofmanufacturing steps.

CROSS REFERENCE TO RELATED APPLICATION

This application is based upon and claims the benefit of priority fromthe prior Japanese Patent Application No. 2005-229,815 filed on Aug. 8,2005, the entire contents of which are incorporated by reference.

1. A thin film piezoelectric resonator comprising: a substrate having acavity; a first electrode extending over the cavity; a piezoelectricfilm placed on the first electrode; and a second electrode placed on thepiezoelectric film, the second electrode having a part of a peripherywhich overlaps on the cavity and is tapered, the second electrode havingan inner angle of 30 degrees or smaller defined by a part of theperiphery thereof and a bottom thereof.
 2. The thin film piezoelectricresonator of claim 1, wherein the part of the periphery of the secondelectrode is tapered and has an inner angle of 15 degrees or larger. 3.The thin film piezoelectric resonator of claim 1, wherein the cavitygoes through the substrate.
 4. The thin film piezoelectric resonator ofclaim 1, wherein the cavity includes an acoustic reflecting layerembedded therein.
 5. The thin film piezoelectric resonator of claim 1,further comprising an insulator which extends over the second electrodeincluding the periphery thereof and the piezoelectric film where thesecond electrode is absent, and has a dielectric constant different froma dielectric constant of the second electrode.
 6. The thin filmpiezoelectric resonator of claim 1, wherein the insulator is made of asilicon oxide film, a silicon nitride film or an aluminum nitride film.7. The thin film piezoelectric resonator of claim 2, further comprisingan insulator which extends over the second electrode including theperiphery thereof and the piezoelectric film where the second electrodeis absent, and has a dielectric constant different from a dielectricconstant of the second electrode.
 8. The thin film piezoelectricresonator of claim 2, wherein the insulator is made of a silicon oxidefilm, a silicon nitride film or an aluminum nitride film.
 9. A thin filmpiezoelectric resonator comprising: a substrate having a cavity; a firstelectrode extending over the cavity; a piezoelectric film placed on thefirst electrode; a second electrode placed on the piezoelectric film andhaving a part of a periphery thereof which overlaps on the cavity, andan insulator placed on the second electrode and the piezoelectric filmwhere the second electrode is absent, and being thin on the center ofthe second electrode and thick on the periphery of the second electrode.10. The thin film piezoelectric resonator of claim 9, wherein theinsulator is thick in a range of 1 μm to 10 μm on the periphery of thesecond electrode, and is thin on the remaining part of the secondelectrode.
 11. The thin film piezoelectric resonator of claim 9, whereinthe insulator is thick in a range of 2 μm to 5 μm on the periphery ofthe second electrode, and is thin at the remaining part of the secondelectrode.
 12. The thin film piezoelectric resonator of claim 9, whereinthe insulator is made of a silicon oxide film, a silicon nitride film oran aluminum nitride film.
 13. The thin film piezoelectric resonator ofclaim 9, wherein an acoustic reflecting layer is embedded in the cavity.14. A thin film piezoelectric resonator comprising: a substrate having acavity; a first electrode extending over the cavity; a piezoelectricfilm placed on the first electrode; a second electrode placed on thepiezoelectric film and having a part of a periphery which overlaps onthe cavity, is tapered, and has an inner angle of 30 degrees or smallerdefined by a part of the periphery thereof and a bottom thereof; and aninsulator placed on the second electrode and the piezoelectric filmwhere the second electrode is absent, and being thin on the center ofthe second electrode and thick on the periphery of the second electrode.15. The thin film piezoelectric resonator of claim 14, wherein theperiphery of the second electrode is tapered and has an inner angle of15 degrees or larger.
 16. A thin film piezoelectric resonatorcomprising: a substrate having a cavity; a first electrode extendingover the cavity; a piezoelectric film placed on the first electrode; asecond electrode placed on the piezoelectric film and having a peripherywhich overlaps on the cavity, and an insulator placed on the secondelectrode and the piezoelectric film where the second electrode isabsent, and having the thickness varying on the piezoelectric film andon the periphery of the second electrode.
 17. A method of manufacturinga thin film piezoelectric resonator, the method comprising: making acavity in a substrate; making a first electrode over the cavity; makinga piezoelectric film on the first electrode; making an electrode forminglayer on the piezoelectric film; making a photo-resist layer on theelectrode forming layer, the photo-resist layer overlapping on thecavity; tapering an edge of the photo-resist layer, the taperedphoto-resist layer having an acute angle and serving as a mask; andpatterning the electrode layer using the mask in order to make a secondelectrode, and transferring a shape of the tapered edge of the mask ontoan end of the second electrode, the tapered edge of the second electrodehaving an acute inner angle.
 18. The method of manufacturing the thinfilm piezoelectric resonator of claim 17, wherein in the mask makingprocess, the photo-resist layer is baked to taper the edge of the resistlayer.
 19. The method of manufacturing the thin film piezoelectricresonator of claim 17, wherein the electrode layer is dry-etched usingthe mask in order to make and taper the periphery of the secondelectrode.
 20. The method of manufacturing the thin film piezoelectricresonator of claim 19, wherein the electrode layer is tapered and has aninner angle of between 15 degrees or larger and 30 degrees or smallerdefined by a part of the periphery thereof and a bottom thereof.